java - Bind two generic types -

i have question regarding generics. have following interfaces:

public interface interfacea<b extends interfacea.interfaceb> {      public interface interfaceb<a extends interfacea> {          void seta(a a);     } } 

and following abstract implementation of interfacea:

public abstract class aimplone<b extends interfacea.interfaceb> implements interfacea<b> {      private final b b;      public aimplone(b b) {         this.b = b;         b.seta(this); // <-- unchecked call...     } } 

its clear me, call b.seta(this) unchecked - don't it, tried second abstract implementation:

public abstract class aimpltwo<a extends interfacea, b extends interfacea.interfaceb<a>> implements interfacea<b> {      private final b b;      public aimpltwo(b b) {         this.b = b;         b.seta((a)this); // <-- unchecked cast     } } 

and again, clear me, call b.seta((a)this) uncheck cast.

but how should implemented or redesigned in order rid of unchecked code?

you having mutual recursive generic definition break using raw types: in

b.seta((a)this); // <- unchecked cast 

this of type interfacea<? extends interfacea.interfaceb<? extends interfacea>>, should of type interfacea<? extends interfacea.interfaceb<? extends interfacea<? extends interfacea.interfaceb<...>>>>. have use instead

public interface interfacea<b extends interfacea.interfaceb<?>> {      public interface interfaceb<a extends interfacea<b>> { //<- cannot make static reference non-static type b          void seta(a a);     } } 

but cannot use b, non-static, in static interface declaration (interface declarations static).

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for detail, 1 further try: using alternative

public interface interfacea<b extends interfacea.interfaceb<?>> {      public interface interfaceb<a extends interfacea<? extends interfacea.interfaceb<?>>> {          void seta(a a);     } }   abstract class aimpltwo<b extends interfacea.interfaceb<a>, extends interfacea<b>> implements interfacea<b> {      private final b b;      public aimpltwo(b b) {         this.b = b;         b.seta((a)this); // <-- unchecked cast     } } 

causes again unchecked cast, since nested type parameter of interfacea in interface interfaceb<a extends interfacea<? extends interfacea.interfaceb<?>>> again arbitrary subclass of interfacea.interfaceb<?>.

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update, since you've asked general design:

i think of interfaceb (in fact, interfaces in general) abstraction concrete implementation: need interface interfaceb, not implementation details, in implementation of interfacea. think of interfaceb contract, , not care implementation. hence there no need binding implementation of interfacea implementation of interfaceb:

public interface interfacea {      public interface interfaceb {          void seta(interfacea a);     } } 

only if, reasons can't see, want have same type instances of interfaceb using, need generics. vice versa interfacea. last generics example above, can @ least fix types interfacea , interfaceb, , have dynamically assert a's b , b's same.

showing no type checked solution exists in java difficult, maybe becomes plausible following example, solution if java allowed combination of extends , super:

public interface a<tb extends a.b<?>> {      public interface b<ta extends a<? extends a.b<?>>> {          void seta(ta a);     } }   class aimpltwo<tb extends a.b<ta>, ta extends aimpltwo<tb, ta> super aimpltwo<tb, ta>> implements a<tb> {      private final tb b;      public aimpltwo(tb b) {         this.b = b;         b.seta((ta)this);     } } 

...come think of it, pluggable type system, adds further typing java, allows combination of extends , super, , might therefore offer solution problem. find complex get, , either stick using interfaces without generics or unchecked cast.